SIPPING Working Group G. Camarillo
Internet-Draft Ericsson
Expires: January 5, 2005 July 7, 2004
Requirements and Framework for Session Initiation Protocol (SIP)Uniform Resource Identifier (URI)-List Servicesdraft-ietf-sipping-uri-services-00.txt
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Abstract
This document describes the need for SIP URI-List Services and
provides requirements for their invocation. Additionaly, it defines a
framework which includes all the SIP extensions needed to meet these
requirements.
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Internet-Draft Reqs and Framework for SIP-URI Services July 20041. Introduction
Some applications require that, at a given moment, a SIP [2] UA (User
Agent) performs a similar transaction with a number of remote UAs.
For example, an instant messaging application that needs to send a
particular message (e.g., "Hello folks") to n receivers needs to send
n MESSAGE requests; one to each receiver.
When the transacton that needs to be repeated consists of a large
request, or the number of recipients is high, or both, the access
network of the UA needs to carry a considerable amount of traffic.
Completing all the transactions on a low-bandwidth access would
require a long time. This is unacceptable for a number of
applications.
A solution to this problem consists of introducing URI-list services
in the network. The task of a SIP URI-list service is to receive a
request that contains or references a URI-list and send a number of
similar requests to the destinations in this list. Once the requests
are sent, the URI-list service typically informs the UA about their
status. Effectively, the URI-list service behaves as a B2BUA
(Back-To-Back-User-Agent).
If the request references an external URI-list (e.g., the Request-URI
is a SIP URI which is associated with a URI-list at the server), this
URI-list is referred to as an stored URI-list. If the request
contains the URI-list, the URI-list is referred to as a
request-contained URI-list.
Stored URI-lists are typically set up using out-of-band mechanisms
(e.g., XCAP [9]). An example of a URI-list service for SUBSCRIBE
requests that uses stored URI-lists is described in [4].
The Advanced Instant Messaging Requirements for SIP [5] mentions the
need for request-contained URI-list services for MESSAGE transactions
"REQ-GROUP-3: It MUST be possible for a user to send to an ad-hoc
group, where the identities of the recipients are carried in the
message itself."
The remainder of this document provides requirements for both stored
and request-contained SIP URI-list services.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
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Internet-Draft Reqs and Framework for SIP-URI Services July 2004
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3. RequirementsSection 3.1 discusses requirements that only apply to
request-contained URI-list services and Section 3.2 discusses
requirements that apply to both stored and request-contained URI-list
services.
3.1 Requirements for Request-Contained URI-List Services
1. The URI-list service invocation mechanism MUST allow the invoker
to provide a list of destination URIs to the URI-list service.
This URI-list MAY consist of one or more URIs.
2. The mechanism to provide the URI-list to the URI-list service
MUST NOT be request specific.
3. The invocation mechanism SHOULD NOT require more than one RTT
(Round-Trip Time).
3.2 General Requirements for URI-List Services
1. An URI-list service MAY include services beyond sending requests
to the URIs in the URI-list. That is, URI-list services can be
modelled as application servers. For example, a URI-list service
handling INVITE requests may behave as a conference server and
perform media mixing for all the participants.
2. The interpretation of the meaning of the URI-list sent by the
invoker MUST be at the discretion of the application to which the
list is sent.
3. It MUST be possible for the invoker to find out about the result
of the operations performed by the URI-list service with the
URI-list. An invoker may, for instance, be interested in the
status of the transactions initiated by the URI-list service.
4. URI-list services MUST NOT send requests to multiple destinations
without authenticating the invoker.
4. Framework
Although Section 3 contains specific requirements for SIP URI-list
services, this framework is not restricted to application servers
that only provide request fan-out services. Per the general
requirement number 1, we also deal with application servers that
provide a particular service that includes a request fan-out (e.g., a
conference server that INVITEs several participants which are chosen
by a user agent).
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Internet-Draft Reqs and Framework for SIP-URI Services July 20044.1 Carrying URI-Lists in SIP
The requirements that relate to request-contained URI-list services
identify the need for a request-independent mechanism to provide a
SIP URI-list service with a URI-list in a single RTT. The mechanism
described in (draft-ietf-sipping-uri-list-00.txt) [3] meets these
three requirements.
UAs (User Agents) use body parts whose disposition type is uri-list
to transport URI-lists. The default URI-list format for SIP entities
is the XCAP resource list format defined in [6].
4.2 Processing of URI-Lists
According to the general requirements 1 and 2, URI-list services can
behave as application servers. That is, taking a URI-list as an
input, they can provide arbitrary services.
So, the interpretation of the URI-list by the server depends on the
service to be provided. For example, for a conference server, the
URIs in the list may identify the initial set of participants. On the
other hand, for a server dealing with MESSAGEs, the URIs in the list
may identify the recipients of an instant message.
At the SIP level, this implies that the behavior of application
servers receiving requests with URI-lists SHOULD be specified on a
per method basis. Examples of such specifications are
[draft-ietf-sipping-uri-list-conferencing-00.txt] for INVITE,
[draft-ietf-sipping-multiple-refer-00.txt] for REFER,
[draft-ietf-sipping-uri-list-message-00.txt] for MESSAGE, and
[draft-ietf-sipping-uri-list-subscribe-00.txt] for SUBSCRIBE.
4.3 Results
According to requirement 6, user agents should have a way to obtain
information about the operations performed by the application server.
Since these operations are service specific, the way user agents are
kept informed is also service specific. For example, a user agent
establishing an adhoc conference with an INVITE with a URI-list may
discover which participants were successfully brought in into the
conference by using the conference package [8].
5. Security Considerations
Security plays an important role in the implementation of any
URI-list service. By definition, a URI-list service takes one request
in and sends a potentially large number of them out. Attackers may
attempt to use URI-list services as traffic amplifiers to launch DoS
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Internet-Draft Reqs and Framework for SIP-URI Services July 2004
attacks. In addition, malicious users may attempt to use URI-list
services to distribute unsolicited messages (i.e., SPAM) or to make
unsolicited VoIP calls. This section provides guidelines to avoid
these attacks.
5.1 List Integrity and Confidentiality
Attackers may attempt to modify URI-lists sent from clients to
servers. This would cause a different behavior at the server than
expected by the client (e.g., requests being sent to different
recipients as the ones specified by the client). To prevent this
attack, clients SHOULD integrity protect URI-lists using mechanisms
such as S/MIME, which can also provide URI-list confidentiality if
needed.
5.2 Amplification Attacks
URI-list services take a request in and send a potentially large
number of them out. Given that URI-list services are typically
implemented on top of powerful servers with high-bandwidth access
links, we should be careful to keep attackers from using them as
amplification tools to launch DoS (Denial of Service) attacks.
Attackers may attempt to send a URI-list containing URIs whose host
parts route to the victims of the DoS attack. These victims do not
need to be SIP nodes; they can be non-SIP endpoints or even routers.
If this attack is successful, the result is that an attacker can
flood with traffic a set of nodes, or a single node, without needing
to generate a high volume of traffic itself.
Note, in any case, that this problem is not specific to SIP
URI-list services; it also appears in scenarios which relate to
multihoming where a server needs to contact a set of IP addresses
provided by a client (e.g., an SCTP [10] endpoint using HEARTBEATs
to check the status of the IP addresses provided by its peer at
association establishment).
There are several measures that need to be taken to prevent this type
of attack. The first one is keeping unauthorized users from using
URI-list services. So, URI-list services MUST NOT perform any request
explosion for an unauthorized user. URI-list services MUST
authenticate users and check whether they are authorized to request
the service before performing any request fan-out.
Note that the risk of this attack also exists when a client uses
stored URI-lists. Application servers MUST use authentication and
authorization mechanisms with equivalent security properties when
dealing with stored and request-contained URI-lists.
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Internet-Draft Reqs and Framework for SIP-URI Services July 2004
Even though the previous rule keeps unauthorized users from using
URI-list services, authorized users may still launch attacks using a
these services. To prevent these attacks, we introduce the concept of
opt-in lists. That is, URI-list services should not allow a client to
place a user (identified by his or her URI) in a URI-list unless the
user has previously agreed to be placed in such a URI-list. So,
URI-list services MUST NOT send a request to a destination which has
not agreed to receive requests from the URI-list service beforehand.
Users can agree to receive requests from a URI-list service in
several ways, such as filling a web page, sending an email, or
signing a contract. Additionally, users MUST be able to further
describe the requests they are willing to receive. For example, a
user may only want to receive requests from a particular URI-list
service on behalf of a particular user. Effectively, these rules make
URI-lists used by URI-list services opt-in lists.
When a URI-list service receives a request with a URI-list from a
client, the URI-list service checks whether all the destinations have
agreed beforehand to receive requests from the service on behalf of
this client. If the URI-list has permission to send requests to all
of the targets in the request, it does so. If not, the URI-list
service rejects the request, indicating in the rejection the set of
targets for which it did not have permission. This allows the client
to request permission for those targets.
DoS amplification would still happen if the URI-list service
automatically contacted the full set of targets for which it did not
have permission in order to request permission. The URI-list service
would be receiving one SIP request and sending out a number of
authorization request messages. In order to avoid this amplification,
the URI-list service must ensure that the client generates roughly
the same amount of traffic towards the URI-list service as the
service generates towards the destinations. Consequently, the
URI-list service MUST require that clients send and individual
authorization request for each destination.
These individual authorization requests sent by the client may or may
not be routed through the URI-list service. In any case, the URI-list
service MUST be informed about the destinations' responses to these
authorization requests in order to authorize requests towards them.
One possible mechanism for clients to send authorization requests to
the destinations is specified in
[draft-rosenberg-sipping-consent-framework-00.txt], which discusses
consent-based communications in SIP. The requirements for
consent-based communications in SIP are discussed in
[draft-rosenberg-sipping-consent-reqs-00.txt]
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Internet-Draft Reqs and Framework for SIP-URI Services July 20045.3 Unsolicited Requests
Opt-in lists should help fighting SPAMMERS. Still, if a URI-list
service is used to send unsolicited requests to one or several
destinations, it should be possible to track down the sender of such
requests. To do that, URI-list services MAY provide information about
the identity of the original sender of the request in their outgoing
requests. URI-list services can use Authenticated Identity Bodies
(AIB) [7] to provide this information.
5.4 General Issues
URI-list services MAY have policies that limit the number of URIs in
the lists they accept, as a very long list could be used in a denial
of service attack to place a large burden on the URI-list service to
send a large number of SIP requests.
The general requirement number 4, which states that URI-list services
need to authenticate their clients, and the previous rules apply to
URI-list services in general. In addition, specifications dealing
with individual methods MUST describe the security issues that relate
to each particular method.
6. Acknowledges
Duncan Mills and Miguel A. Garcia-Martin supported the idea of 1 to n
MESSAGEs. Jon Peterson and Dean Willis provided useful comments.
7. References7.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
7.2 Informational References
[3] Camarillo, G., "Providing a Session Initiation Protocol (SIP)
Application Server with a List of URIs",
draft-camarillo-sipping-uri-list-01 (work in progress),
February 2004.
[4] Roach, A., Rosenberg, J. and B. Campbell, "A Session Initiation
Protocol (SIP) Event Notification Extension for Resource
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Internet-Draft Reqs and Framework for SIP-URI Services July 2004
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